Aqueous solutions of etchants have long been used to remove thin films from substrates. However, there are a number of disadvantages of wet etching techniques. These disadvantages are especially apparent in the semiconductor industry where wet cleans and etches contribute to particulate and other contamination. Therefore, processes that use gaseous etchants are preferred.
Another undesirable feature of both wet and gaseous etches is that generally the etching is isotropic, and a controlled source of etchant designed for removal of thin layers is not used. Often, only a precise amount of the layer should be etched because there are features that need to be retained without damage. For instance, uncontrolled isotropic etching contributes to the formation of undesirable gaps when the material to be etched is sandwiched between two layers which are resistant to etching. For instance, as shown in FIG. 1, layer 1 and substrate 3 are resistant to etching and layer 2 is readily etched by the isotropic etching process. If an opening is created in layer 1, then isotropic etching of layer 2 produces a gap 4 in layer 2 underneath the opening in layer 1. It is clear, therefore, that what is needed is a controlled etching system which is able to etch the precise thickness of the layer of interest without overetching and accentuating the gap between unetched layers.
One of the most important cleaning and etching steps in the semiconductor industry is to remove native oxide from the surface of silicon. Oxide etching by immersion in aqueous solutions containing HF can yield pristine Si surfaces chemically passivated by hydrogen atom termination of silicon dangling bonds. Chemical passivation reduces reoxidation of silicon upon exposure to air, but it does not reduce contamination by particulates or large organic molecules. Integration of an aqueous silicon dioxide removal process with an evacuated, multichamber wafer processing tool designed to minimize particulate contamination is difficult. Hence, a process that takes place in the gas phase at low pressure, instead of in a liquid, is needed.
Silicon dioxide has been successfully etched or removed from the surface of a silicon wafer by a number of methods that do not use aqueous solutions and which take place with medium or low pressures of reactive molecules or ions. Examples include reactive ion etching (RIE), and heating the wafer to 900.degree. C. temperature for the reaction Si(s)+SiO.sub.2 (s)=SiO(g).sub.4. However, these techniques do not etch by reacting with the layer of material to be etched and a condensed film containing the reactant molecules. Therefore, they are not considered prior art and will not be discussed.
Prior art generally concerns reactions where films are present on a surface during reaction. Some art does not recognize the existence of films on the surface. Other art recognizes the existence of films and the role they play in etching, but does not recognize the importance of the films, nor how to properly control them. Inaccuracy and lack of control in the prior art often arise when there is inadvertent transition between regimes, as described herein infra.
In etching of silicon dioxide, prior art has reported different etch rates for TEOS, thermal silicon dioxide, and other types of silicon dioxide. The rate is limited by the reactivity of the reactant toward silicon dioxide. The reactivity varies according to structural and compositional differences in the silicon dioxide. The rate of passage of reactant through a film did not limit the rate of reaction with these oxides.
U.S. Pat. No. 4,264,374 to Beyer and Kastl discloses a way to use gaseous vapor to remove native oxide from silicon. Oxide etching by exposure to gaseous mixtures of HF, H.sub.2 O, and N.sub.2 near atmospheric pressure has very high selectivity for removal of silicon dioxide without removing silicon. As the pressure of a process increases, problems from contamination by particulates and gaseous impurities increase. Hence, a lower pressure process is needed.
The equipment described by Deal et. al (J. Appl. Phys. 36:3370, 1965) and Novak (Solid State Technology, 31:39, 1988) is currently available and is designed to etch oxide layers using gaseous mixtures containing HF and H.sub.2 O. However, the equipment is not designed to control the surface residence time, the thickness or composition of condensed films in the etching system. An inert gas is bubbled through water or through a solution of HF in water in a container called a bubbler. The pressure of water vapor or HF and water vapor entrained in the inert gas is equal to the vapor pressure at the temperature of the bubbler. Flow controllers regulate admission of the gases into an unheated chamber holding the substrate. The bubblers and gas lines are heated. In equipment described by Deal et al supra, the substrate can be heated, and the total pressure can be regulated by altering the pumping speed of the system. There is no quartz crystal microbalance in the system, and the substrate cannot be cooled. Etching is restricted to high chamber pressures. It is a flow system with no capability to isolate the reaction chamber from the vacuum pump.
Another method for removing oxide is mentioned by Nishino et al (Proceedings of the Symposium on Dry Process, Inst. of Elec. Eng. of Japan, Tokyo, Oct. 30, 1989, p. 90) in a report on microwave discharge cleaning of silicon. Exposure of silicon covered with a native oxide to gases from a heated container filled with ammonium fluoride or to a microwave discharge in NH.sub.3 and NF.sub.3 gas followed by raising the temperature of the silicon can remove the oxide. In a separate experiment, HF and sulfuric acid were formed following activation of precursor molecules in a discharge. Nishino et al does not deal with the type of gap illustrated in FIG. 1, nor with any other undesired effect induced by overetching, etc.
In summary, prior art apparatus for etching with condensed films containing reactant is not designed to control the composition and residence time of the film. The following items are missing in prior art equipments: a monitor of thin films such as a microbalance, a means to maintain the wafer as the coldest point in the chamber, separate gaseous sources of NH.sub.3 and HF, sources of NH.sub.3, H.sub.2 O, and HF, an ammonium bifluoride source, and low pressure sources such as effusion cells, or sources with a differentially pumped region, there is no ability to maintain the chamber wall temperature above the temperature of any reactant source which is a condensed liquid or solid, the vacuum pump cannot be isolated from the chamber, and there is no ability to simultaneously open/shutoff valves between gas sources and chambers.
The design of the prior art etching systems is necessarily dependent on the devices and methodologies known at the time and can be understood by considering the design of available systems, such as reactive ion etching systems, used for etching a surface when the reactant does not condense on the surface. The rate at which reactant strikes the surface to be etched is proportional to the pressure of the reactant in the chamber. As long as the reactant pressure is below its vapor pressure at the temperature of the substrate, reactant may adsorb on the surface for a short period of time, but is does not condense to form a multilayer. The pressure of reactant, and therefore the reaction rate, is controlled by varying the rate at which the reactant is admitted to the chamber and the rate at which the reactant is removed from the chamber, e.g., by the pump attached to the chamber. The admission rate of reactant to the chamber can be regulated with a flow meter. This type of equipment is also used in the prior art to carry out reactions when the reactant condenses on the surface of the substrate to be etched even though initiation and control of the reaction is very different when a condensed reactant film forms on the surface to be etched. The differences are more fully set forth below.
When a condensed reactant film forms on the surface, the composition of the condensed film is more important than the composition of the gas phase. The partial pressure of reactant in the gas phase is not as important because the gaseous reactant is not in direct contact with the surface to be etched. Instead, the condensed reactant film is in contact with the surface, and therefore the composition of the condensed reactant film is more important than the composition of the gaseous ambient in determining how fast the surface will be etched, and how much of the surface will be etched. The composition of the gas affects the reaction only indirectly as gaseous reactants are transported through the condensed film to the surface. A central problem is that condensed multilayer films can form on surfaces other than the layer to be etched, such as chamber walls or areas of the wafer-mount which may be colder than the surface of the wafer. The layers that form on these extraneous surfaces can serve as a source or sink for reactant so that there is no longer a definable relationship between the composition and timing of gases admitted to the chamber and the composition and residence time of reactant layers on the surface of the wafer to be etched.
Deal et al, supra, mention the presence of an aqueous film on the surface of the oxide during vapor phase etching, but do not recognize the importance of controlling the film. Instead, the systems described by Deal and Novak use flowmeters to admit reactant to the chamber. Referring to FIG. 2a, the operation of the prior art equipment may be described as follows: nitrogen from reservoir A passes through mass flow controllers B and into a bubbler C containing water or bubbler D containing a mixture of HF and water. After passing through the bubbler, the gas enters chamber E where a wafer F is mounted on a room temperature holder G. In some equipment, the gas and reaction products pass from chamber E through a regulating valve H before being pumped away.
Although suitable for etching reactions not involving a layer of condensed reactant, this system can cause difficulty in any reaction which occurs when condensed reactant forms on the surface of the layer to be etched. When a flowmeter admits gas to a chamber at a constant rate, the pressure rises slowly until the pressure is high enough for the chamber vacuum pump to remove the gas at the same rate at which it is admitted to the chamber. This slow pressure rise contributes to an ambiguous onset time for condensation, an indefinite film thickness and an ambiguous composition.
The formation of a film on a surface from constituents in the gas phase is known to be regulated by the temperature of the surface and the partial pressures of the gaseous constituents. Once the partial pressure of the gaseous constituents rises to a value equal to or greater than the vapor pressure, a condensed film will form. If the pressure rise is slow, the onset time for condensation is ambiguous. Once condensation begins, the partial pressure of the gaseous constituents of the film will remain equal to the vapor pressure of the constituents. Condensation continues, and the thickness of the condensed film increases because the system pump is unable to remove the constituents of the film at the same rate at which they are admitted.
Control of the extent of film removal by the apparatus described by Deal et. al. and by Novak, supra, is therefore difficult. Since a flow controller is used, the partial pressures of water and HF in the chamber rise slowly after initiation of flow. Thus, the time at which the condensation occurs and the reaction is initiated is not well defined. The thickness of the film increases with time. The composition of the gas above the film will change with time in a complex fashion because as the reaction proceeds (SiO.sub.2 +4HF=SiF.sub.4 +2H.sub.2 O), HF is consumed and H.sub.2 O is produced as a byproduct. Thus, the surface area of oxide being etched can affect the gaseous composition. Finally, condensation on the walls of the chamber serves as an additional unpredictable source or sink for HF because the gaseous HF will exchange with HF in the condensed film on the walls of the chamber.
Exchange of constituents in the gas with constituents in the condensed film can be understood by referring to FIG. 2b from p. 148 of Physical Chemistry by Farrington Daniels and Robert Alberty, 3rd ed. 1966, Wiley, N.Y. It shows how the equilibrium composition of the vapor of a two component mixture of benzene and toluene varies from the composition of the liquid as a function of temperature. Similar curves hold for all binary mixtures including HF/H.sub.2 O. The x axis shows the fractional composition of the mixture and the y axis shows the temperature. If vapor is admitted to a chamber with the composition of point b, it will condense on an object whose temperature is below about 94.degree. C. The composition of the vapor, represented by point d, is very different than the composition of the liquid represented by point c. In other words, as soon as condensation occurs, re-evaporation will alter the composition of both the condensed film and the gas phase.
The exact details of how these compositions change with time will have a complex dependence on the absolute and relative amount of condensed film on the walls and on the substrate, flow rates, absolute pressures etc. The absolute amount and relative amount of condensed film on the walls of the chamber and on the substrate will also depend on the ambient temperature and on the temperature difference between the substrate and the chamber. The actual curve for HF/H.sub.2 O binary mixtures shows the existence of an azeotrope. Azeotropes are certain fractional compositions of a binary mixture where the vapor, and liquid in equilibrium with the vapor, have the same composition. Although Deal et. al. mention use of the azeotrope in a bubbler as a supply of HF and water, they combine this with a bubbler containing water so that the gaseous composition inside the chamber is not the composition of the azeotrope.
U.S. Pat. No. 5,030,319 to Nishino et. al. provides an understanding of etching mechanisms where films that contain reactant are used for etching. However, the etching apparatus and method are in some ways more difficult to use despite an improved understanding of the films. For instance, a flow system is used to admit gases with all the potential problems mentioned above. In addition, reactant is created through reaction of precursor molecules activated by a plasma discharge instead of direct admission of the reactant. The formation of condensed multilayer reactant films, therefore, depends not only on temperatures, flow rates, pumping speeds, etc.; but also on the characteristics of the discharge and the reaction of activated precursor molecules. Furthermore, reactive radicals or molecules in the gas phase can consume, react with, or chemically transform films on the wafer or walls.
The Nishino patent teaches that a discharge in NH3 and NF3 forms HF which can combine with ammonia to form ammonium fluoride layers that react with SiO.sub.2 to form an ammonium hexafluorosilicate product. Reaction can also occur by dissolution of reactant within the product layer. The results indicate that not all the silicon dioxide which reacts is left behind as a film on the surface of the oxide. FIG. 2 of the Nishino patent shows the etching rate and film thickness for a 10 minute etching time as a function of the NH3 to NF3 ratio. The etching rate of around 10-40 Angstroms/min corresponds to removal of 100 to 400 Angstroms of silicon dioxide. If all the reacted silicon dioxide were converted to ammonium hexafluorosilicate product resident on the surface, the thickness would be approximately 300 to 1200 Angstroms, far thicker than the film thicknesses shown in Nishino's FIG. 2. Some of the silicon from the reacted silicon dioxide does not remain resident on the surface in the form of ammonium hexafluorosilicate. When the product film is thin, it can be "broken away", but after continued etching it can no longer be "broken away" and etching is terminated. The termination thickness of etched oxide in FIG. 15 of Nishino patent is about 1000 Angstroms, a thickness far larger than needed to prevent undercut shown in FIG. 1 of this application. The decrease in etching efficiency is presented as a problem which can be remedied by alternating several short reaction and product desorption cycles instead of etching for one long period of time.
If a thin film is formed with a short 10 minute discharge, the film remains permeable to reactant, if hydrogen is substituted for ammonia in the discharge HF is formed and in fact the presence of the film increases the reaction rate. The film does not inhibit the reaction.
Nishino further teaches that radicals such as O.sub.2 and fluorine atoms in the discharge influence the reactions in several ways. They can affect the selectivity of etching silicon vs. silicon dioxide. When H.sub.2 SiO.sub.4 and HF are formed in the discharge, fluorine atoms are consumed within the condensed film when it is thick, but fluorine atoms are not consumed when the condensed film is thin. When the film is thin, silicon is etched; when the film is thick, silicon is not etched. These reactions can complicate controlled etching.
It may be further noted that any apparatus using a condensed source of reactant at a temperature hotter than the wafer surface is able to make a transformation between regimes. If reactant is admitted slowly, there is no reaction initially, because an adsorbed film does not form. As the pressure of reactant slowly increases to near the vapor pressure at the temperature of the wafer, then an adsorbed film can form which etches the surface. The adsorbed film composition is closely related to the composition of the gases in the chamber. When the pressure exceeds the vapor pressure of the material in the chamber, there is condensation of a multilayer. The multilayer forms a reservoir, especially if it is a liquid multilayer, the composition of which is no longer directly connected to the composition of the gases in the chamber. No prior art system is designed and operated to stay in the regime where an adsorbed film of a layer or less is responsible for etching.
Furthermore, constant slow rate admission of reactant with flow controllers to a chamber using apparatus mentioned in the prior art does not lead to accurate etching or removal of thin layers, because the composition and residence time of the reactant film is not controlled: e.g., the time at which a condensed film of reactant is formed on the surface is uncertain, the length of time condensed film remains on the surface is unknown, condensed film can form on the walls of the chamber or portions of the wafer mount which are colder than the wafer, the fraction of the reactant which is admitted to the chamber and eventually condenses and reacts is unknown, the substrate is not cooled sufficiently to facilitate use of a low pressure source, the self limiting reaction thickness of solid products is uncertain, a wafer is not held at or slightly above the temperature of a source of condensed reactant so that reaction can occur within the adsorbed film regime, and the presence of activated radicals or molecules can complicate the composition of films.
It is clear from the above that improvements in the prior art apparatus and method are needed.